Magnetotransport properties and magnetostructural phenomenon in single crystals of ${\mathrm{La}}_{0.7}({\mathrm{Ca}}_{1-y}{\mathrm{Sr}}_y{)}_{0.3}{\mathrm{MnO}}_3$

The magnetotransport and magnetostructural properties of ${\mathrm{La}}_{0.7}({\mathrm{Ca}}_{1-y}{\mathrm{Sr}}_y{)}_{0.3}{\mathrm{MnO}}_3$ $(0\mathrm{}<~y<~1)$ are studied. We investigate the variation in magnetoresistance (MR) with y and the field-induced structural transition at $y\sim 0.5$ by using single crystals prepared by the floating-zone method. With the increase in y, the colossal MR behavior at $y\sim 0$ is transformed to a rather canonical MR (a MR free from lattice effects) at $y=1.\mathrm{}$ The coefficient C in the empirical relation that $\rho \mathrm{}\left(M\right)/\rho \mathrm{}\left(0\right)=\exp \left[-{C(M/M}_s{)}^2\right]$ (M and $M_s$ being the magnetization and its zero-temperature saturation value) shows a significant change with y, attainable for example, from 7.6 at $y=0.1\mathrm{}$ to 2.3 at $y=\mathrm{0.7.}\mathrm{}$ As an example of the magnetoswitching of a crystal structure around room temperature, the structural-phase transition induced by an external magnetic field is investigated for crystals with $y\sim \mathrm{0.5.}$ Structural-phase diagrams of the magnetic field vs temperature plane are given.